I asked Ed what he thinks of TEPCO's comment quoted in the New Scientist: ". . . for the fuel pond at reactor 4, 'the risk of recriticality is not zero', meaning a nuclear chain reaction could restart in the rods. Quite how this has come about is unclear."
Ed agrees this is unclear. Very unclear. Well nigh impossible. He wrote QUOTE: Let me explain the situation in more detail. Two kinds of fuel are being used, uranium metal clad by zirconium (Unit, 1,2, and 4) and UO2+PuO2 clad with zirconium (Unit 3). The spend fuel is heated by radioactive decay and must be actively cooled using flowing water. The new fuel is not hot. Once power failed, the spent rods started heating the water that remained in the ponds after being shaken by the earthquake. Spent fuel in several of the ponds got hot enough boil water and react with the remaining water to produce H2, which exploded and blew out the buildings. The shock wave from the explosion removed the rest of the water. Water reacts with hot Zr to produce powdered ZrO2 and with uranium to produce powdered U3O8, which generates a lot of heat, resulting in the observed fires. The UO2 is only slightly reactive with water and is not the problem. The uranium has a melting point near 1130°C, which I doubt can be reached by heat from radioactive decay. In any case, the rods in the ponds are in supports that contain neutron absorbers that would prevent a critically event. Even if the uranium melted, it would react rapidly with air and not form a molten blob. In other words, a blob of metal simply cannot form outside of the isolation provided by the reactor wall. Once the rods are hot enough to react with water, trying to cool them using water is the worst approach because formation of U3O8 will release all the fission products in the uranium. The melting point of UO2+PuO2 fuel (about 2100° C) is too high to be reached under any condition outside of a reactor and is best allowed to cool naturally. At the present time, the best approach is to do nothing. This will not be done because this would admit defeat and would have to be explained to people who simply do not understand the situation. Consequently, the water will eventually release most of the fission fragments and then the long cleanup will begin. END QUOTE I asked what about the possibility that the fuel rod cladding has burned or split, and the pellets are in a pile of debris at the bottom of the dry pond. He said that is possible, but a pile of pellets still will not go critical. The geometry will not support that. They would have to be in a compact mound right on top of one another. Also the spent fuel "contains a lot of poisons, which is why it was removed from the reactor." Meaning it does not react easily. That would not apply to the fuel removed temporarily, to be reshuffled and put back. - Jed
